Genetic carrier screening plays a role in family planning by providing information about inherited conditions that could be passed on to children. This proactive step allows individuals and couples to make informed decisions regarding their reproductive health.
Understanding Spinal Muscular Atrophy (SMA)
Spinal Muscular Atrophy (SMA) is a genetic disorder that impacts nerve cells, known as motor neurons, located in the spinal cord and brainstem. These motor neurons control voluntary muscle movement in areas such as the arms, legs, face, chest, throat, and tongue. When motor neurons are lost, muscles do not receive the necessary signals to move, leading to progressive muscle weakness and wasting, often more pronounced in the trunk and upper limbs.
SMA is primarily inherited in an autosomal recessive pattern. This means that for a child to develop the condition, they must inherit two altered copies of a specific gene, typically one from each parent. The most common form of SMA is caused by a mutation in the SMN1 (survival motor neuron 1) gene, which is responsible for producing the SMN protein necessary for motor neuron health. The severity of SMA symptoms can vary, influenced by the number of copies of the SMN2 gene, which produces a small amount of functional SMN protein.
What is an SMA Carrier Screen?
An SMA carrier screen is a genetic test designed to identify individuals who carry one altered copy of the SMN1 gene. These individuals, known as carriers, typically do not exhibit symptoms of SMA because their other SMN1 gene copy functions properly. The purpose of this screening is to determine if individuals, particularly those planning a pregnancy, are at risk of passing on the SMA gene mutation to their children.
The test is usually performed using a simple blood sample. In the general population, approximately 1 in 50 people are carriers of the abnormal gene that causes SMA. The American College of Obstetricians and Gynecologists (ACOG) recommends offering SMA carrier screening to all women who are pregnant or considering pregnancy. Screening is also suggested for individuals with a family history of SMA or a known SMA carrier.
Interpreting Carrier Screening Results
The outcomes of an SMA carrier screen generally fall into two categories: negative or positive. A “negative” result indicates that the common SMA gene mutations were not detected, significantly lowering the chance of having a child with SMA. However, a small residual risk remains.
A “positive” result signifies that you are identified as a carrier of an SMA gene mutation. This means you carry one non-working copy of the SMN1 gene. Being a carrier does not mean you have SMA or will develop it; rather, it indicates that you could pass the altered gene to your offspring. If only one parent is a carrier, the risk of their child developing SMA is low, though there is a 50% chance the child will also be a carrier.
Navigating a Positive Carrier Result
If an individual receives a positive SMA carrier screen result, the next step often involves testing their partner. If both partners are identified as SMA carriers, there is approximately a 1 in 4 (25%) chance with each pregnancy of having a child affected with SMA.
Genetic counseling becomes an important resource for couples facing a positive result. Genetic counselors help explain the inheritance patterns, risks, and available reproductive options. These options can include prenatal diagnosis, such as chorionic villus sampling (CVS) or amniocentesis, to determine the fetal SMA status. Preimplantation genetic diagnosis (PGD) in conjunction with in vitro fertilization (IVF) is another option, allowing embryos to be screened for the SMA mutation before implantation.
Other considerations might involve using donor gametes or adoption. Individuals should discuss these pathways thoroughly with their healthcare provider and a genetic counselor to make choices aligned with their family planning goals.